Lung function depends on the ease with which air passes from the atmosphere to the alveoli (air sacs) and back to the atmosphere again. This is mainly determined by the flow resistance of the small airways of the lungs. Lung function can vary considerably over short periods of time, and can be affected by such factors as temperature, humidity, exercise and disease, such as asthma. For example, a person playing a sport may suddenly become short of breath as a result of a sport-induced bronchiospasm, in which the bronchial tubes contact due to exertion. Asthma sufferers are also particularly vulnerable to allergens, viruses and smoke.
Although portable peak-flow meters are available at low price, their usefulness in analyzing lung function is limited. Such peak-flow meters are prone to significant errors, which are particularly undesirable in the case where the meters are used to regulate a drug treatment for a pulmonary disorder. Known low price peak-flow meters are based on mechanical friction spring arrangements, which are uncalibrated, and intended to provide relative results only. Such devices are often used to regulate a user's intake of steroids, which provide a preventative treatment for asthma. It will be appreciated that an incorrect reading from the peak-flow device will result in the user's taking an incorrect dose of steroids, either too much or too little, which is undesirable and potentially dangerous.
In order to carry out accurate pulmonary function tests, it is normally necessary for the patient's lung function to be tested in the hospital using non-portable testing equipment. However, in view of the fact that asthma, particularly in the case of asthma suffers who are children, is believed often to contain a psychosomatic element, tests which are carried out in the hospital do not always give representative results. This is because the mere fact that a patient has to attend the hospital tends to increase the stress level of the patient, and this in turn leads to less reliable test results. Lung function is very changeable over short periods of time, e.g. before and during exercise. To get a complete picture of a person's lung function, multiple measurements over time need to be taken during normal day-to-day activities. At present, testing a patient in the hospital is often the only way of carrying out the full range of appropriate tests.
Such tests result in a variety of useful measurements, the five most significant of which are summarized below.
PEAK FLOW is the simplest measurement, and is simply an indication of the peak velocity of expelled air expressed in liters per minute. This measurement, like the other measurements discussed below, is typically obtained by asking the patient to blow into suitable apparatus.
VC is the total volume of expelled air, expressed in liters.
FEV1 is the volume of air expelled in the first second, expressed in liters.
FEV1/VC is the volume of air expelled in the first second divided by the total volume expelled, expressed as a percentage.
FEF25%-75% is the average velocity of air flow between 25% expelled volume and 75% expelled volume, expressed in liters per minute.
Devices for carrying out the above measurements typically involve the patient's blowing through a tube containing a restriction, and taking pressure measurements on each side of the restriction. The restriction is often in the form of one or more gauzes or meshes, which have the effect of reducing the chaotic behavior of the air flow through the pipe. A portable device which operates on the basis of pressure measurements on either side of a flow restriction is described in Australian Patent Application No. 67994/90. FIG. 1 of that application shows a hand-held device (see FIG. 1), the upper part of which is provided with a straight tube 16 along which the user blows air in order to obtain PEAK FLOW and FEV1 measurements. These measurements are obtained by measuring the pressure on either side of a restriction within the tube 16, as shown in FIG. 5. Although the device is portable, the device cannot easily be carried in a pocket. This is due in part to the length of the tube 16 of the manufactured article, which is around 12 cm. In the prior art, lengths of this size or greater have been favored in order to reduce the chaotic behavior of the air flow within the tube, thus making reliable pressure measurements within the tube more easy to carry out.
The invention seeks to provide an improved personal pulmonary function analyzer, and an improved method of performing flow measurements in a personal pulmonary function analyzer.